For retracting gingiva from a prepared tooth a cord can be used. In this respect, a retraction cord is packed between gingival tissue and the margin of the prepared tooth (this region is also often called sulcus) using an appropriate dental instrument e.g. a Heinemann spatula. To obtain sufficient vertical and horizontal retraction of gingival tissue, it is often necessary to pack several lengths of retraction cord into the sulcus in order to be able to make a detailed dental impression.
A description of the background in regard to retraction cords can be found e.g. in U.S. Pat. No. 4,522,593.
U.S. Pat. No. 4,871,311 describes a retraction method using a retraction cord made at least in part of a swelling material.
Generally, dental retraction cords are sometimes difficult to place into the gingival sulcus. The procedure can also be time consuming. It can also be cumbersome to remove the retraction cord prior to taking the impression. Coagulated blood may adhere to the cord and removing it may open the wound again which results in bleeding.
For a more convenient placement retraction pastes have been suggested.
Non-hardening retraction pastes containing either an anti-evaporating component or fibrillated fibers are described in e.g. US 2005/0008583 and US2005/00287494.
U.S. Pat. No. 5,362,495 refers to a method for widening the gingival sulcus without bleeding or oozing, comprising inserting within the gingival sulcus a material in the form of a biocompatible paste which is injectable for external use and having a plastic viscosity measured at 20° C. between about 13,000 and 30,000 Pa*s, wherein said material consisting of a material selected from the group of white clay, seaweed meal and mixtures thereof.
A similar approach is described in JP 2006056833. The paste consists of an astringent and filler containing clay mineral, torque, mica, kaolin and/or montmorillonite.
A commercially available product to be used for retraction is sold under the name Expasyl™. However it is reviewed, that Expasyl™ is only effective under specific, limited conditions when the sulcus is flexible and of sufficient depth. The paste's thickness makes it difficult for some evaluators to express it into the sulcus. Moreover, according to the instruction of use, the viscosity of the composition might change when fluids like water, saliva or blood are absorbed. However, a reduction in viscosity is sometimes undesirable since having high consistency could in certain circumstances be a pre-requisite for applying force onto the gingiva for sufficient retraction.
Generally, removing non-hardening pastes completely out of the sulcus before taking the impression can be very time consuming and cumbersome. Usually, the paste is rinsed off using water-spray. However, sometimes paste residues are located deep in the sulcus and are thus difficult to remove. These residues might prevent the impression material from flowing into the sulcus area and may negatively influence the setting of the impression material which is subsequently applied. Moreover, after rinsing off the paste with water an additional drying step might be required before the impression can be taken. These removing and drying steps could cause bleeding of the tissue and might make an impression taking step more complicated.
Hardening materials are sometimes easier to remove. However, they are not very hydrophilic. This might cause problems with regard to flowability of the material into the gingival sulcus.
US 2004/0106086 describes an impression material which can be used for retraction of gingival tissue.
US 2005/0260543 describes a method or retracting gum tissue comprising the steps of placing an initially flow able material around a tooth, covering the tooth and flow able material with a dam made of a porous material, applying pressure to the dam and removing the dam with the set material.
US 2005/0069838 discloses a dental kit and method for retraction sulcus using an expanding silicone compound or mixture of different silicone compounds. However, silicone compounds are of inorganic and hydrophobic nature, thus having limited biocompatibility with oral tissue and disadvantages in flowing to moist tissue and tooth surfaces and moist areas like the gingival sulcus.
DE 37 37 552 A1 suggests to use alginates, hydrocolloids or silicone for dental retraction. No hints as to the nature of the materials to be used for the retraction procedure are given. It is only mentioned that some kind of reaction should take place (expansion by chemical reaction and/or physical swelling of the material) and that the material can be incorporated into a dental impression material selected from alginate, hydrocolloids and silicones, materials which cure after application.
Impression materials based on alginates are usually delivered in a powdery form which can form an irreversible hydrocolloide in the presence of water. The powder typically contains potassium or sodium alginic acid, filler(s), retarder(s) and additives. The pastes are made either by hand-mixing the powder and water or by using special mixing devices. Dental alginate impression materials usually have high filler content (generally above about 60 wt.-% with respect to the whole composition in dry form, that is, before water is added). Despite of this high filler content the set alginate materials have limited tensile strength because of their gel-like consistency after cure and thus are not suitable for use as a retraction material. Moreover, the setting time of this type of material is sometimes deemed to be too long for retraction procedure (e.g. above about 2 or 3 min).
Thus, there is still a need for an improved dental retraction composition. Ideally, a material is desired, which can be cured in the patients' mouth at a time convenient for the practitioner (e.g. on demand).